The present invention relates to a method for determining a position according to the preamble of the appended claim 1. The invention also relates to an electronic device according to the preamble of the appended claim 10, and a positioning system according to the preamble of the appended claim 17.
One known positioning system is the GPS system (Global Positioning System) which presently comprises more than 20 satellites. Some of these are out of sight, below the horizon, at a time when viewed from a positioning receiver, wherein the signal of these satellites cannot be received in practice. The satellites transmit e.g. Ephemeris data as well as data on the time of the satellite. A receiver used in positioning normally deduces its position by calculating the propagation time of a signal transmitted simultaneously from several satellites belonging to the positioning system to the receiver. For the positioning, the receiver must typically receive the signal of at least four satellites within sight to compute the position.
Each satellite operating in the GPS system transmits a ranging signal at a carrier frequency of 1575.42 MHz called L1. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. In the satellite, the modulation of these signals is performed with at least one pseudorandom sequence. This pseudorandom sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. The modulation technique used makes it possible in the receiver to distinguish between the signals transmitted from different satellites, although the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L1 signal, the pseudorandom sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with a polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to produce different C/A codes with an identical code generator. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the iteration time (epoch) of the code is 1 ms. The carrier of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the xe2x80x9chealthxe2x80x9d, orbit, time data of the satellite, etc.
During their operation, the satellites monitor the condition of their equipment. The satellites may use for example so-called watch-dog operations to detect and report possible faults in the equipment. The errors and malfunctions can be instantaneous or longer lasting. On the basis of the health data, some of the faults can possibly be compensated for, or the information transmitted by a malfunctioning satellite can be totally disregarded. Furthermore, in a situation in which the signal of more than four satellites can be received, different satellites can be weighted differently on the basis of the health data. Thus, it is possible to minimize the effect of errors on measurements, possibly caused by satellites which seem unreliable.
To detect the signals of the satellites and to identify the satellites, the receiver must perform acquisition, whereby the receiver searches for the signal of the satellite and attempts to be synchronized and locked to this signal so that the data transmitted with the signal can be received and demodulated. After the acquisition, the receiver attempts to keep locked, or to track the signal of the satellite at least during the time of positioning, but in some cases, the tracking phase can be maintained as long as the receiver receives the signal of the satellite sufficiently strongly.
The positioning receiver must perform the acquisition e.g. when the receiver is turned on and also in a situation in which the receiver has not been capable of receiving the signal of any satellite for a long time. Such a situation may easily occur for example in portable devices, because the device is moving and the antenna of the device is not always in an optimal position in relation to the satellites, which reduces the strength of the signal coming to the receiver. Also, in urban areas, buildings affect the signal to be received, and furthermore, so-called multipath propagation can occur, wherein the transmitted signal comes into the receiver along different paths, e.g. directly from the satellite (direct line-of-sight) and also reflected from buildings. This multipath propagation causes that the same signal is received as several signals with different phases.
The positioning arrangement has two primary functions:
1. to calculate the pseudorange between the receiver and the different GPS satellites, and
2. to determine the position of the receiver by utilizing the calculated pseudoranges and the position data of the satellites. The position data of the satellites at each time can be calculated on the basis of the Ephemeris and time correction data received from the satellites.
The distances to the satellites are called pseudoranges, because the time is not accurately known in the receiver. Thus, the determinations of position and time are iterated until a sufficient accuracy is achieved with respect to time and position. Because time is not known with absolute precision, the position and the time must be determined e.g. by linearizing a set of equations for each new iteration.
The pseudorange can be calculated by measuring the pseudo transmission time delays between signals of different satellites. After the receiver has been synchronized with the received signal, the information transmitted in the signal is determined.
After the code acquisition has been completed, the next steps are frequency tuning and phase locking. This correlation result also indicates the information transmitted in the GPS signal.
The above-mentioned acquisition and frequency control process must be performed for each signal of a satellite received in the receiver. Some receivers may have several receiving channels, wherein an attempt is made on each receiving channel to be synchronized with the signal of one satellite at a time and to find out the information transmitted by this satellite.
The positioning receiver receives information transmitted by satellites and performs positioning on the basis of the received information. For the positioning, the receiver must receive the signal transmitted by at least four different satellites to find out the x, y, z coordinates and the time data, if none of this information is available for use by the receiver in a sufficiently reliable way. In some cases, it is possible to transmit, e.g. from a base transceiver station, the height data of the base station, wherein for the positioning it is sufficient that the receiver receives the signal transmitted by three satellites. Inaccuracies of a few meters in the height direction do not significantly impair the positioning accuracy. The received navigation information is stored in a memory, wherein of this stored information e.g. satellite position information can be used.
FIG. 1 shows, in a principle chart, positioning in a positioning receiver MS by means of a signal transmitted from four satellites SV1, SV2, SV3, SV4. In the GPS system, the satellites transmit Ephemeris data as well as time data, on the basis of which the positioning receiver can perform calculations to determine the position of the satellite at a time. These Ephemeris data and time data are transmitted in frames which are further divided into subframes. In the GPS system, each frame comprises 1500 bits which are divided into five subframes of 300 bits each. Since the transmission of one bit takes 20 ms, the transmission of each subframe thus takes 6 s, and the whole frame is transmitted in 30 seconds. The subframes are numbered from 1 to 5. In each sub-frame 1, e.g. time data is transmitted, indicating the moment of transmission of the subframe as well as information about the deviation of the satellite clock with respect to the time in the GPS system.
The subframes 2 and 3 are used for the transmission of Ephemeris data. The subframe 4 contains other system information, such as universal time, coordinated (UTC). The subframe 5 is intended for the transmission of almanac data on all the satellites. The entity of these subframes and frames is called a GPS navigation message which comprises 25 frames, or 125 subframes. The length of the navigation message is thus 12 min 30 s.
In the GPS system, time is measured in seconds from the beginning of a week. In the GPS system, the moment of beginning of a week is midnight between a Saturday and a Sunday. Each subframe to be transmitted contains information on the moment of the GPS week when the subframe was transmitted. Thus, the time data indicates the moment of transmission of a certain bit, i.e. in the GPS system, the moment of transmission of the last bit in the subframe. In the satellites, time is measured with high-precision atomic chronometers. In spite of this, the operation of each satellite is controlled in a control centre for the GPS system (not shown), and e.g. a time comparison is performed to detect chronometric errors in the satellites and to transmit this information to the satellite.
At the stage when the positioning receiver is turned on, it must first perform synchronization to at least four satellites to determine the position and the time data. Furthermore, the positioning receivers can perform acquisition and positioning at intervals, wherein possible moving of the positioning receiver can be taken into account in the positioning. If the positioning receiver has position information (almanac data) of the satellites, rough time data (precision on the minute level is usually sufficient) and its own position available at some accuracy, the positioning receiver can usually find out which satellites are in sight at a time. If any of the above-mentioned items of information is missing or is incorrect, the positioning receiver does not know which satellites are in sight. Thus, the positioning receiver must try to find out a sufficient number (at least four) of satellites in sight and perform acquisition. Thus, the positioning receiver can, however, try to perform acquisition also to a signal of a satellite which is out of sight. This increases the time taken on positioning and, on the other hand, also the total power consumption of the positioning receiver. There are positioning receivers known in which acquisition to a satellite is attempted in a certain order, for example in the order of the satellite indices. However, this solution has the draw-back that a lot of time may be spent on finding a sufficient number of satellites in sight, because the predetermined order of searching is not optimal in all situations.
It is an aim of the present invention to provide a method for determining the position of a receiver. It is also an aim of the invention to provide an electronic device comprising a positioning receiver in which the method is applied. The invention is based on the idea of determining the directional angles between the satellites in a preferably Earth centered coordinate system and selecting a first satellite whose signal is attempted to receive. If the signal of the selected satellite is detectable, the signal of the satellite closest to this satellite is next searched for. However, if the signal of the first selected signal is not detected, the satellite selected to be searched next is the satellite farthest from the first selected satellite. The method is iterated until a sufficient number of satellite signals has been received. Directional angles are used in this invention as an indicator of the distance between the satellites. The method according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 1. The electronic device according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 10. Further, the positioning system according to the present invention is characterized in what will be presented in the characterizing part of the appended claim 17.
Considerable advantages are achieved by the present invention when compared with methods and positioning receivers of prior art. When applying the method according to the invention, the positioning can be speeded up, because it is possible with the method to quickly eliminate such satellites which are very probably located below the horizon in view of the positioning receiver. When applying the method of the invention, it is possible to significantly speed up the detection of the satellites required for the positioning when compared with systems of prior art; consequently, also the total power consumption of the electronic device comprising the positioning receiver can be reduced, which is particularly useful in portable electronic devices.